Circuit for dynamic compression and/or expansion

Abstract

In a circuit for the automatic dynamic compression or expansion of a signal, the circuit presenting a useful signal path containing a first variable gain member which is electronically controllable for varying the transmission factor between the circuit input and output, a branch path connected at one point to the useful signal path and containing a control voltage generator for generating a control voltage from the signal at such point in the useful signal path, and a second variable gain member which is connected in the branch path and which is electronically controllable for varying the transmission factor exhibited by the branch path, with the transmission factor of the useful signal path being controlled to have a value dependent on the value of the control voltage and a counterregulation of the transmission factor of the branch path being effected relative to the transmission factor variation imparted to the useful signal path, the useful signal path includes a device causing the signal at one point along that path to have a value proportional to the logarithm of the signal at the circuit input, the branch path has its input point connected to the one point of the useful signal path, each variable gain member has an antilogarithmic transmission factor, and the output of the control voltage generator serves to vary the gain of each said variable gain member as an exponential function of the control voltage.

What is claimed is: 
     
       1. In a circuit for the automatic dynamic compression or expansion of a signal between the input and output of the circuit, the circuit presenting a useful signal path connected between its input and output and containing a first variable gain member which has a control input and which is electronically controllable in response to a first control signal at its control input for varying the transmission factor between the circuit input and output, the circuit also presenting a branch path having an input point connected to the useful signal path and containing a control voltage generator for generating a control voltage from the signal at such point in the useful signal path, a second variable gain member which is connected in the branch path and which has a control input and which is electronically controllable in response to a second control signal at its control input for varying the transmission factor exhibited by the branch path, and means connecting the output of the control voltage generator to the variable gain members supplying respective first and second control signals thereto for causing the transmission factor of the useful signal path to have a value dependent on the value of the control voltage and for effecting a counterregulation of the transmission factor of the branch path relative to the transmission factor variation imparted to the useful signal path, the improvement wherein: said useful signal path includes means causing the signal at one point along said path to have a value proportional to the logarithm of the signal at said circuit input;   said branch path has its input point connected to said one point of said useful signal path;   each of said variable gain members constitutes means having a transmission factor which is antilogarithmic and which varies as an exponential function of the control voltage; and   said means connecting the output of the control voltage generator serve to cause the first and second control signals to have respectively different values.   
     
     
       2. Circuit as defined in claim 1 wherein each said variable gain member comprises a bipolar transistor connected such that its collector current changes, in dependence on its base-emitter voltage, according to the exponential function. 
     
     
       3. Circuit as defined in claim 2 wherein each said variable gain member comprises one npn transistor and one pnp transistor, said two transistors are connected to receive respectively opposite polarities of the control voltage, the emitters of said transistors are connected to receive the useful signal, and the collector currents of said two transistors are added and constitute the output signal of said first variable gain member. 
     
     
       4. Circuit as defined in claim 3 wherein the collectors of said transistors are connected together, and further comprising an adder circuit having an input connected to the connected-together collectors of said transistors. 
     
     
       5. Circuit as defined in claim 3 wherein said means connecting the output of the control voltage generator comprise two voltage dividers associated with each said variable gain member and connected between said control voltage generator and the bases of said transistors of the associated variable gain member. 
     
     
       6. Circuit as defined in claim 5 wherein said first and second variable gain members are of identical design. 
     
     
       7. Circuit as defined in claim 6 wherein the dividing ratios of said voltage dividers of each said variable gain member are identical. 
     
     
       8. Circuit as defined in claim 7 wherein the dividing ratios of said voltage dividers of said first variable gain member differ from the dividing ratios of said voltage dividers of said second variable gain member by a constant factor. 
     
     
       9. Circuit arrangement as defined in claim 3 wherein the emitters of said transistors receive the useful signal for compensation of the base-emitter voltages of said transistors of said variable gain members offset with respect to one another by a constant voltage value. 
     
     
       10. Circuit arrangement as defined in claim 1 wherein said means connecting the output of the control voltage generator cause the values of the first and second control signals to have a constant ratio.